1. Field of the Invention
The present invention relates to a tone generating apparatus for use in an electronic musical instrument, which generates musical tones. More particularly, this invention pertains to a tone generating apparatus which can generate conventionally-available musical tones although the number of digital controlled oscillators (each hereinafter referred to as DCO) is reduced.
2. Description of the Related Art
Conventional tone generating apparatuses for use in electronic musical instruments, such as an electronic organ and an electronic piano, each have multiple DCOs as tone generating sources. Some of these DCOs are properly combined and are driven to generate tone signals. The combination is determined in accordance with, for example, the timbre designated through an operation panel, the tone range specified through a keyboard, etc.
More specifically, the tone generating apparatus includes the same number of tone generating circuits as the number of polyphonic sounds (the number of simultaneously-generatable tones). One tone generating circuit corresponds to one key. Each tone generating circuit has four DCOs 1 to 4 which respectively generate tone signals of individual components, attack, decay, high and noise. The four oscillators, DCO 1 to DCO 4, are simultaneously operable when one key is depressed.
The attack component, decay (attenuating sound) component and high (hard hitting sound) component, which constitute one tone, have a relatively long tone-generating time or tone-ON time. The frequencies of these tone components vary in proportion to the pitch. The noise (striking sound) component has a relatively short tone-ON time, and has a frequency that should not necessarily be proportional to the pitch.
The mentioned tone components are respectively generated by the four oscillators DCO 1 to DCO 4. The generated four tone components undergo a predetermined arithmetic operation to be synthesized together, thus yielding a single tone signal.
Musical tones generated by the thus constituted tone generating apparatus become closer to the tones of a natural musical instrument.
The conventional tone generating apparatus, as explained above, has the same number of oscillators for a noise component as the number of polyphonic sounds, in order to generate striking sounds having a short tone-ON time, as well as oscillators for the other tone components having a long tone-ON time. At the time a musical tone is generated, a channel is assigned to generate the striking sound as is done for generation of the other tone components having a long tone-ON time.
However, the striking sound does not vary in accordance with the pitch, i.e., the position of a depressed key. Further, the striking sound will attenuate immediately after it is generated for a short period of time. To provide the same number of oscillators for generation of the noise component as the polyphonic number and assign a channel to the noise generation requires a great amount of hardware and results in a complicated structure. This means that the tone generating apparatus becomes expensive.